1. Field of the Invention
The present invention relates to an optical scanning apparatus used in an image forming apparatus such as a copying machine, printer, facsimile machine, or a multifunction peripheral.
2. Description of the Related Art
An optical scanning apparatus used in an electrophotographic image forming apparatus deflects a beam of light emitted from a light source with an optical deflector, and condenses the light beam towards a surface to be scanned with a scanning and image forming optical system to thereby form an optical spot on the surface to be scanned. An optical scanning apparatus is well known that is configured to form a latent image on a surface to be scanned by executing an optical scan of the surface to be scanned with the optical spot.
This type of scanning and image forming optical system generally includes at least one fθ lens. An fθ lens is a lens designed to provide a flat field on the surface of the image plane to be scanned. The fθ lens generally adopts a specific lens effective surface such as an aspherical surface for the purpose of improving scanning characteristics.
A resin lens is often used as the fθ lens due to advantages including simple formation of the above type of specific surface and cost-effective manufacture. In particular, since many lenses are used in a tandem image forming apparatus, considerable cost reduction is realized by use of resin lenses.
On the other hand, when compared to a glass lens, a resin lens tends to undergo thermal expansion due to increased temperature, and consequently, unintended distortion of light rays may occur.
An optical deflector is often realized by a rotational polygon mirror that rotates a deflection reflection surface that is driven by a rotational driver.
When driving the deflection reflection surface of the rotational polygon mirror, temperature increase may result from the heat generated by the driving components such as a motor. Thus when the optical scanning is continuously executed over a long period of time, the bearing portion of the rotation shaft for the rotational polygon mirror or the IC chips in the motor unit for driving the rotational polygon mirror may reach a substantially high temperature.
Even during short optical scanning operations, since temperature increase varies as a logarithmic function immediately after starting the rotation, a distorting temperature increase in an inner portion of the optical scanning apparatus has a large effect on the distortion of light rays.
The fθ lens and the rotational polygon mirror forming the optical scanning apparatus are housed in an inner portion of an optical box. When the temperature of the rotational polygon mirror or the electronics for controlling the motor increases, the resulting heat creates a convection current of hot air in the optical box and the hot air is transmitted to the optical box as convective heat. In addition, a flow of high-temperature air created by the rotation of the rotational polygon mirror impacts on the fθ lens, at least partially.
Consequently, a relative temperature difference is generated on the fθ lens, and the resulting partial thermal expansion causes a distortion of the optical surface. As a result, a distortion in the scanning light flux is produced, and causes a lack of uniformity in a scanning speed or image forming characteristics to cause deterioration in the image such as unevenness or streaking. Particularly, in a tandem image forming apparatus a problem occurs in which deviations of positions, in product colors, are generated caused by changes in the position of a light beam for each color.
To solve the above problems, an optical scanning apparatus has been proposed that reduces the effect of heat produced by the rotational polygon mirror. For example, the optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 9-197330 is configured in such a manner that a glass cover or the like covers a deflecting device or an optical member so that heat from a deflecting device or the like is not transmitted to an optical member. The optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 9-197330 considers a light beam that scans a single light-sensitive drum.
However, an optical scanning apparatus of an opposed-scanning system, as illustrated in FIGS. 5A and 5B, that deflects and scans laser light L1 and L2 to expose a plurality of light-sensitive drums using a single polygon mirror 51 entails additional problems.
Specifically, when the polygon mirror 51 is rotated, a flow of air is generated. The air flow collides against a vertical wall 53 provided to prevent flare light from entering an optical lens 52, and is divided into air flows F1 and F2. The air flow F1 is further divided into air flows F3 and F4 by an optical lens 54 positioned on an upper portion of the polygon mirror 51.
Since the air flow such as the air flow F3 or F4 that contains heat generated by rotation of the polygon mirror 51 diffuses in the optical scanning apparatus, the heat distribution in the optical scanning apparatus is reduced. In this manner, thermal expansion of a part in the optical scanning apparatus is suppressed and color deviation can be effectively avoided.
However, the optical lens 54 may undergo thermal expansion as a result of the air flow F3 colliding against the laser light incidence plane 55 of the optical lens 54.
To solve this problem, when a method discussed in Japanese Patent Application Laid-Open No. 9-197330 is used in a configuration in which heat is not transmitted to an optical lens, since an optical flux passes through optical members including glass that is not required to condense the optical flux on the surface to be scanned, the laser beam is affected by a distortion or the like of the surface thereof. As a result, there is a high possibility of problems including distortion in the scanning beams or failure of the optical spot.